1. Field of the Invention
The invention generally relates to the detection of defects or faults, such as bubbles, inclusions, cracks, etc., in ribbons of brittle or fracture-prone material which is at least partially transparent, when the ribbon is being or has been wound into a coil. More particularly, the invention relates to the localization and classification of defects such as cracks in rolled glass ribbons or glass coils.
2. Description of Related Art
In the manufacturing of ultra-thin glasses, i.e. glasses having a thickness of less than or equal to 200 μm, the highly flexible glass ribbon is subjected to strong vibrations, due to the high speed of the moving glass ribbon and the drawing of the so-called glass films at a high drawing rate, such vibrations being of an extent so as to go beyond the small focus range of high resolution optics. A result therefore is increased slip and increased misclassification of detected defects. Furthermore, it is observed that when a ribbon that has been detected as error-free glass is wound into a coil, the ribbon in the coil may break as a result of stresses in the edge region caused by the bending operation or due to an increase in the surface load on the coil. If an edge previously has been damaged with microcracks, smallest tensile stresses may cause breakage of the glass. Microcracks may for instance be caused either inherently by the molding, cooling, and transfer processes, or by the cutting of the border, i.e. the edge region of the glass ribbon.
The detection of cracks in the glass ribbon coil is crucial for transport and further processing of the glass ribbon, for example for outgoing inspection at the producer, or for incoming inspection at the customer. As part of quality inspection it is for example important to determine how many cracks exist in the coil and at which positions and what is the length of the defect-free glass ribbon between the cracks. With the knowledge of crack positions, it can be prevented that previously damaged glass or glass residues or glass splinters are introduced into the systems for further processing.